The Effect of NaCl Content in Saline Water on the Performance and Microstructure of Ni-YSZ Cermet Electrodes of Solid Oxide Electrolytic Cells

Abstract

Hydrogen production via seawater electrolysis using solid oxide electrolytic cells (SOECs) exhibits significant potential for sustainable development. Therefore, an in-depth understanding of the adverse effects of seawater in particular the NaCl content on the performance and microstructure of nickel-yttria-stabilized zirconia (Ni-YSZ) electrodes of SOECs is highly necessary. This study systematically investigates the influence of NaCl solutions with different concentrations (simulating seawater and industrial high-salinity wastewater) as steam sources on the performance and long-term stability of SOECs at 800 °C. The results show that the NaCl content has no significant impact on the initial electrochemical performance of SOECs (e.g., current density, ohmic resistance). Nevertheless, the NaCl content is particularly detrimental to the interface region of the cell. During electrolysis or under open-circuit conditions, NaCl can react with Ni to form NiCl2, which accelerates migration and causes delocalized agglomeration of the Ni phase in the fuel electrode in addition to localized agglomeration of Ni via the Ni(OH)2 route. The delocalized agglomeration of the Ni phase is supported by the detection of Ni in the off-gas condensate of the cells in NaCl solutions. The combination of localized and delocalized agglomeration causes detachment of the Ni phase from the YSZ phase of the cermet, leading to a reduction in triple-phase boundaries (TPB) and activity degradation. This accelerating effect is more pronounced under electrolysis conditions in the presence of NaCl in saline water. With prolonged operation, the continuous loss of Ni, the reduction in TPB, and the additional interface reactions induced by NaCl collectively exacerbate the performance and microstructure decay of Ni-YSZ cermet electrodes of SOECs.